The integration of LSI has been remarkably improved in recent years, and the width of a circuit pattern line is accordingly becoming much finer. LSI has a diffusion-preventing layer formed to prevent the reaction diffusion between aluminum forming electrodes and silicon forming elements. With the circuit pattern line width becoming finer, this diffusion-preventing layer is also required to have a smaller thickness and further required to have a high melting point highly sufficient to prevent the diffusion. Further, since the diffusion-preventing layer constitutes part of an electrode, it is preferred to select a material therefor from materials having lowest possible, specific resistance. A titanium nitride layer is presently attracting attention as a material having a high melting point and low specific resistance and having a remarkably excellent diffusion-preventing effect.
The above titanium nitride layer is formed by a reactive sputtering method using pure titanium as a target (Monthly Semiconductor World, 1992, 3p56). In this reactive sputtering method, an impact is applied to a target of pure titanium by means of charged particles of an nitrogen ion and an argon ion formed by glow discharging, whereby not only the target is nitrided, but also titanium nitride particles are released by the force of the impact to form a titanium nitride film on a silicon wafer opposed to the target.
Further, there is a recently proposed method in which a target of a titanium nitride compound having a nitrogen/titanium atomic ratio of 1:1 is manufactured and the sputtering is carried out with only an argon ion to form a titanium nitride film (U.S. Pat. No. 4,820,393).
However, the above sputtering methods have the following problems. The problem of the former reactive sputtering method using pure titanium as a target is that it is difficult to convert the entirety of titanium to titanium nitride when titanium nitride is formed from nitrogen introduced into a sputtering apparatus and the titanium target. Under some conditions, sputtered particles which are physically driven out by charged particles come to contain unreacted titanium.
When sputtered particles contain unreacted titanium, the resultant thin film contains residual, unreacted titanium, and the unreacted titanium in the thin film and an aluminum thin film formed as a circuit pattern react with each other, which ends up in the deterioration of the intended diffusion prevention, so-called barrier properties.
Since the above unreacted titanium has higher specific resistance than titanium nitride, the unreacted titanium causes an increase in the specific resistance of the thin film.
Furthermore, when pure titanium is used as a target for sputtering, the nitrogen/titanium compositional ratio of the sputtered particles keenly varies relative to the partial pressures of inert gases such as nitrogen and argon introduced to a sputtering apparatus, the pressure of ambient atmosphere and the electric power inputted to the apparatus, and the composition of the resultant titanium nitride thin film also varies. It has been therefore necessary to bring the sputtering conditions under control very accurately for obtaining a low-resistance thin film whose nitrogen/titanium atomic ratio (hereinafter referred to as N/Ti) is substantially 1.
In addition to these problems, the reactive sputtering method using pure titanium as a target involves another problem that the N/Ti differs between a middle portion and a marginal portion of a film on the wafer, i.e., nonuniformity in the film composition.
The above-described problems caused when pure titanium is used as a target are inherent to an attempt to introduce the entirety of nitrogen required to constitute the composition of a thin film in the form of a nitrogen gas.
On the other hand, in the method in which a titanium nitride thin film whose composition agrees with a target composition is formed from a titanium nitride target having an N/Ti atomic ratio, i.e., N/Ti, of 1, there is a problem in that a large number of huge particles, so-called "particles", occur on the resultant titanium nitride thin film to break an electrode pattern line.
The cause for the occurrence of the above particles is considered as follows. When an attempt is made to form a target from a stoichiometric amount of titanium nitride which has N/Ti of substantial 1 and gives low resistance, this titanium nitride so poor in sinterability since its melting point is very high, as high as 3,290.degree. C., that it is difficult to increase the density of a titanium nitride compound target. That is, fine pores are present in the target, and abnormal discharging occurs during the sputtering.
There is another problem in that the target undergoes chipping at a production step or during use due to the fragility of the stoichiometric titanium nitride in the target or low strength in bonding among stoichiometric titanium nitride grains.
The above problems are all due to poor producibility of a target having N/Ti=1 in composition.
Further, there is an attempt to form, by ordinary sputtering, a thin film whose composition agrees with a target composition from a target having a composition of TiNx in which x is 0.1 to 1.0 (Japanese Laid Open Patent No. 63-259075).
However, the above target is ultimately used for forming a film having the same TiNx composition as that of the target, but is not used for forming a film by using a reaction with a nitrogen gas. That is, the above target is used for nonreactive sputtering. For forming a barrier metal layer required to have low resistance, x in TiNx is required to be substantially 1, and after all, the above target has some problems of the stoichiometric titanium nitride target that the sinterability is very poor.